Because many Germans have a huge inferior complex and need football to compensate for it.
That's also why even people like Uli Hoeneß many years ago were not ashamed to publicly announce that Real Madrid would soon go bankrupt.
The world cup final was a nightmare for many German football fans as it meant that either Messi was going to be crowned or France was adding one more world cup to their title list and thus getting closer to Germany.
If they are not successful on the pitch, then at least they need to feel like they are superior when it comes to solid finances.
Btw, ever noticed that German fans will usually even cheer for their archenemy in Europe? Could you ever imagine a die-hard United fan rooting for Liverpool or Manchester City in the Champions League? In Germany that behavior is really common as it's the German football fans biggest dream to see the Bundesliga dominate European football.
9/10, if you want to go full Brexiteer you should've somehow included WW2 in there to get a perfect result